How do I auto normalize in grease pencil weight painting
The checkbox is located under the brush settings.



Also look at the manual page:


  Auto Normalize
  
  Ensures that all deforming vertex groups add up to one while painting. When this option is turned off, then all weights of a vertex
  can have any value between 0.0 and 1.0. However, when Vertex Groups
  are used as Deform Groups for character animation then Blender always
  interprets the weight values relative to each other. That is, Blender
  always does a normalization over all deform bones. Hence in practice
  it is not necessary to maintain a strict normalization and further
  normalizing weights should not affect animation at all.
  
  This option works most intuitively when used to maintain normalization while painting on top of weights that are already
  normalized with some other tool.